This application claims priority to and the benefit of Australian Provisional Patent Application Number PQ8489, which was filed in Australia on Jun. 30, 2000.
This invention relates to a pinch roll assembly for feeding strip material that is particularly useful at high temperatures where the strip cannot be quenched during feeding. It has application in feeding hot metal strip produced from a continuous caster such as a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls. The casting rolls are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between the casting rolls to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels from where the molten metal flows through a metal delivery nozzle forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
The hot strip leaving the caster may be passed to a coiler on which the strip is wound into a coil. Between the caster and the coiler the strip may be subjected to in-line treatment such as a controlled temperature reduction, reduction rolling, full heat treatment or a combination of such treatment steps. The coiler and any in-line treatment apparatus generally applies substantial tension to the strip. Moreover, differences between the casting speed of the twin roll caster and speed of subsequent in-line processing and coiling must be accommodated. Substantial differences in those speeds may develop particularly during initial start-up and until steady state casting speed is achieved. To accommodate these requirements, the hot strip leaving the caster may be allowed to hang unhindered in a loop form and then passed through one or more sets of pinch rolls into a tensioned part of the line in which the strip may be subjected to further processing before coiling. The pinch rolls provide resistance to the tension generated by the down-line equipment and are also intended to feed the strip into the down-line equipment.
A twin roll strip casting line of this kind is disclosed in U.S. Pat. No. 5,503,217 assigned to Davy McKee (Sheffield) Limited. In this casting line the hot metal strip hangs unhindered in a loop before passing to a first set of pinch rolls which feed the strip through a temperature control zone. After passing through further sets of pinch rolls, the strip then proceeds to a coiler. The strip may optionally be hot rolled by inclusion of a rolling mill between the subsequent sets of pinch rolls. However, as noted in U.S. Pat. No. 5,503,217, strip passing from zero tension to a tensioned part of a processing line can wander from side to side. This wandering of the strip may be overcome by providing a first set of pinch rolls to steer the metal strip from the loop into the tensioned part of the processing line.
This first set of pinch rolls must be capable of gripping and feeding the hot metal strip very soon after it has solidified. Particularly when casting ferrous metal strip, the strip temperature at this position in the line is very high, more than 1000xc2x0 C. and typically of the order of 1200xc2x0 C., and the strip itself will be very soft and easily damaged. Furthermore, the strip at this location is enclosed in a reducing atmosphere where quench water cannot be applied to the strip as it is fed through the pinch rolls. It has been found that if conventional steel pinch rolls are used for feeding the hot strip at this position localized defects are imprinted in the surface of the strip that appear in the finished strip. Under these conditions, the imprinted defects are generally due to the generation of hot spots on the steel pinch rolls with resultant localized thermal expansion at those regions and production of projections which imprint depressions in the strip surface. When rolling steel strip in this process, scale from the strip surface can stick to the high spots on the pinch rolls. Accordingly, any high spots due to localized thermal expansion can rapidly be built up to substantial projections which can produce severe imprint defects in the strip.
The present invention enables this problem to be alleviated by providing a pinch roll assembly that reduces generation of high spots and reduces the formation of projections on the roll surfaces due to localized thermal expansion. According to the invention, there is provided a pinch roll assembly for feeding hot metal strip that is comprised of a pair of parallel pinch rolls to receive the strip in the nip between the pinch rolls, and drive means to drive the pinch rolls so as to feed the strip between the pinch rolls. At least one of the pinch rolls, and may be both, is comprised of a pair of end support shafts, a cylindrical tube of copper or copper alloy extending between the support shafts, and cooling water passages to enable cooling water to flow internally of the roll to cool the sleeve. The cylindrical tube provides an external peripheral roll surface of at least 300 mm in diameter, and together with the cooling water passages, and resulting cooling water flow, are sufficient to provide small displacement of the strip at the nip of the pinch rolls.
The end shafts are connected to a cylindrical arbour (i.e., a solid or hollow cylindrical frame) to which the copper or copper alloy tube is fitted as an external sleeve. In this embodiment, the water flow passages may be confined to the cylindrical arbour. More specifically, the cooling water passages may include longitudinal passages in the cylindrical arbour spaced, typically evenly, circumferentially around the arbour adjacent the sleeve.
Alternatively, the roll may be of an arbourless construction in which the end shafts have end formations connected to respective ends of the cylindrical tube of copper or copper alloy. In this embodiment, the water flow passages may deliver cooling water to the interior of the cylindrical tube or the passages may extend longitudinally through the tube.
The diameter of the external peripheral roll surface of the pinch roll is may be at least 500 mm. Alternatively, the diameter of the external peripheral roll surface of the roll may satisfy the following equation:                     D         greater than                   2          ⁢                                    σ              y                              -                2                                      ·            q                    ⁢                      xe2x80x83                    ⁢                      1                          π              ⁡                              (                                                                            1                      -                                              v                        1                        2                                                                                    E                      1                                                        +                                                            1                      -                                              v                        2                        2                                                                                    E                      2                                                                      )                                                                        (        1        )            
where
q: Load per unit width
D: Pinch roll diameter
v1, v2: Poisson""s ratio of roll and strip
E1, E2: Young""s modulus of roll and strip
"sgr"y:Minimum yield stress
The invention may be used with apparatus for continuously casting metal strip comprising a pair of casting rolls forming a nip between them, a metal delivery means for delivery of molten metal into the nip between the casting rolls to form a casting pool of molten metal supported on the casting roll surfaces immediately above the nip, roll drive means to drive the casting rolls in counter rotational directions to produce a solidified strip of metal delivered downwardly from the nip, and strip feed means disposed generally to one side of the caster to receive strip from the caster and feed it away from the caster. The pinch roll assembly of the present invention may be used to apply tension to the hot strip shortly after casting at high temperature above 1000xc2x0 C. in an enclosed chamber with a reducing atmosphere.
The pinch roll assembly mean comprises a pair of parallel pinch rolls to receive the strip in the nip between the rolls, and drive means to drive the roll so as to feed the strip between the pinch rolls. At least one and usually both of the pinch rolls comprises a pair of end support shafts, a cylindrical tube of copper or copper alloy extending between the support shafts to provide an external peripheral roll surface, and cooling water passages internally to the roll to cool the tube by flow of cooling water. The pinch roll assembly may be a pair of end support shafts, a cylindrical arbour to which the copper or copper alloy tube is fitted as an external sleeve or an arbourless cylindrical sleeve of copper or copper alloy extending between the support shafts to provide the external peripheral roll surface. The external diameter of the peripheral roll surface is more than 300 mm, and together with the cooling water passages, and resulting cooling water flow, enables a small displacement of the strip at the nip of the pinch rolls.